Biological carbon dioxide (CO2) fixation by non-photosynthetic microbes holds a great promise for producing useful products (e.g., chemicals and biofuels) from CO2 (which requires an electron source as well, such as H2). Among the non-photosynthetic carbon fixation pathways, the most important one is the Wood-Ljungdahl (WL) pathway in that it is the only linear pathway known to fix carbon. This is practically significant in that CO2 can serve as the sole carbon source as long as there is an electron source, such as H2. The WL pathway is employed by anaerobic organisms, most of which are of the Clostridia class, and are broadly known as acetogens that they fix CO2 and H2 to produce acetate. These organisms are responsible for the production of about 10% of the 100 billion U.S. tons of acetic acid which is produced annually on earth. Thus, they fix billions of tons of CO2 on the earth every year and produce 10 billions tons of acetate. Clostridia are Gram-positive, strict anaerobic endospore-formers, are mostly soil organisms, and belong to the phylum Firmicutes. The Clostridia class includes many families, genera (including Clostridium and Moorella), and is made up of hundreds of known species.
The WL pathway is found in over 100 species of anaerobic bacteria, many of which are in the class Clostridia. The best known among them are the acetogenic Clostridia, such as Clostridium aceticum, Clostridium difficile, C. ljungdahli and Moorella thermoacetica (formerly Clostridium thermoaceticum) and, also, Acetobacterium woodii. Besides a strong medical interest in these organisms, Clostridia, with or without a native WL pathway, have been the organisms of choice for the biological production of solvents and butanol. For example, C. acetobutylicum, though without a full and functional WL pathway, is a Gram-positive obligate anaerobe that is well-known industrially for its ability to produce commodity chemicals (e.g., butyrate, acetate, acetoin, and acetone) and biofuels (e.g., butanol and ethanol).
There remains a need to engineer Clostridia and other anaerobes without a functional WL pathway to improve their metabolism of CO2 as a carbon source for production of useful chemicals and biofuels.